Aircraft turbojet engines generate significant amounts of noise pollution. There is a strong demand for this pollution to be reduced, especially since the turbojet engines used are becoming increasingly powerful. The design of the nacelle that surrounds a turbojet engine plays a significant part in reducing this noise pollution.
In order further to improve the acoustic performance of aircraft, nacelles are provided with acoustic panels the purpose of which is to attenuate the noise generated by the turbojet engine and the vibrations of the structures.
Acoustic panels are structures that are well known for absorbing these noises. These panels conventionally comprise one or more layers of cellular core structure (structure commonly known as “honeycomb”). These layers are generally coated on their lower surface, that is to say on their face not in contact with the flow of air through the nacelle, with an air-impermeable or “solid” skin and, on their upper surface, that is to say on the face in contact with the flow of air through the nacelle, with an air-permeable perforated skin known as the “acoustic” skin.
The acoustic panel may further comprise several layers of cellular core structure between which there is inserted, for example bonded, a porous or multi-perforated skin known as a “septum”.
Such panels constitute acoustic resonators capable of “trapping” the noise and therefore of attenuating the noise emitted to the outside of the nacelle.
In a known way, the cellular core structure is created from (a) joined cellular unit(s) known as (a) “honeycomb block(s)”. A cellular unit is generally obtained by superposing several sheets of metal, of light alloy or of composite, on which bonding means are positioned at discrete spots to bond the sheets together at certain points known as joining pads. The cellular unit thus produced is said to be in “compacted” form. In order to form the alveolar cells, the unit undergoes a stretching operation to separate the sheets which remain joined together at the joining pads. The cellular unit thus produced is then said to be in “expanded” form.
The acoustic properties of the acoustic panel, that is to say its noise absorption as a function of the frequency and acoustic level of the noise, are dependent notably on the joining of the cellular unit or units, which form a cellular core structure.
The lateral ends of cellular units are commonly joined using a foaming adhesive, such as the adhesive FM 410®, which has a high ability to expand. Adhesive, generally in the form of a film, is inserted between the adjacent edges of two cellular units and, as it expands, blocks the alveolar cells, creating additional thicknesses at the cell partitions in the joining region. These additional thicknesses have the disadvantage of reducing the effective acoustic area of the cellular structure and also of causing sharp breaks in impedance which play a part in reducing the acoustic performance of the acoustic panel by redistributing the modal energy of the noise through the rotating parts when the turbojet engine is operating.
Use of such a cellular core structure is complicated and does not allow fully uniform acoustic treatment to be obtained.